This invention relates to prediffused integrated circuits in which a network of identical cells is prepared beforehand, said cells being subsequently interconnected on demand for carrying out desired logic functions. The invention is also concerned with methods for interconnecting the integrated circuit cells.
The initial technique for the fabrication of integrated circuits consists in utilizing a desired logic circuit and in transposing the logic functions of said circuit to an array of interconnected transistors so as to form an elementary cell. These cells are subsequently interconnected in order to perform the logic function of the desired circuit.
This method, developed for elementary gates, has proved to be impracticable when the degree of complexity of the logic function to be integrated becomes excessive. Very-large-scale integrated circuits in use at the present time in fact contain tens of thousands of transistors. It can therefore be readily understood that the length of time required in order to implant this large number of transistors as well as their corresponding connecting leads is wholly prohibitive.
Interest has therefore focused on systems based on computer-assisted designs involving the use of standardized cells stored in the library of a computer system. Interconnection of said cells may be carried out, at least with assistance, by means of wiring algorithms processed by a computer.
Two techniques based on this principle are in current use. Their advantages and disadvantages are complementary.
The so-called standard cell technique consists in utilizing a library of relatively complex logic functions placed on the available surface of a substrate while taking into account the necessary interconnections between them. Position-location of the cells takes the interconnection into account and conversely, the maximum substrate area available for integration can thus be utilized by placing the cells and the interconnections with maximum packing density. On the other hand, each integrated circuit is completely specific, thus entailing the need for a complete set of masks which is specific to said circuit in order to permit fabrication. Any modification which proves necessary during development of the circuit is thus liable to affect all the masks which in that case have to be re-made in their entirety. Similarly, in order to obtain a modified circuit, the entire fabrication process has to be begun again after the masks have been modified, which thus takes a relatively considerable length of time. The gain in substrate surface area is thus offset by a loss of production time.
The other technique, known as the prediffused network technique, consists in constructing very simple arrays of cells all identical with each other and aligned in a series of rows separated by free spaces of predetermined width.
The logic function to be integrated is implemented in a final step of the fabrication process by deposition of a network of aluminum connecting leads through a mask which is specific to the desired logic function. As will be readily understood, this network of interconnections must conform to the topological limits established by the arrangement of rows of prediffused cells. Since the space allowed for the interconnections is necessarily limited, this leads to saturation of the passages reserved for the connecting leads in the event of even slight complexity of the function to be integrated. Under those conditions, there remain unused cells which cannot be connected and the net result is a loss of useful surface area of the substrate. Conversely, there is a gain in speed of execution by virtue of the fact that it is no longer necessary to provide more than two or three specific masks and that the fabrication of the circuit calls only for deposition of an aluminum interconnection grid on a substrate comprising all the cells, which is the easiest and fastest step of the fabrication process. The long and difficult steps have been performed beforehand at the time of mass production of the prediffused networks which can be made available from stock.